Activation of complement (C) with consequent generation of the C5-derived peptides C5a and C5a des Arg is a primary chemotactic stimulus at sites of infection or injury. Excessive C5a- mediated leukocyte recruitment into various tissues is known to be a critical step in the pathogenesis of many inflammatory disorders. C5-derived peptides are very potent chemoattractants for almost the entire cellular repertoire of the immune system, and consequently are an attractive target for therapeutic intervention. But the regulation of C5a chemotactic activity in a physiological setting is poorly understood. Previously, several groups have shown that the vitamin D binding protein (DBP), also known as Gc-globulin, can significantly enhance the chemotactic activity of C5-derived peptides, i.e., function as a chemotactic cofactor. This activity of DBP is specific for the C5-derived peptides. However, its role in vivo has not been determined and the mechanism of chemotaxis enhancement is not known. It is our HYPOTHESIS that DBP functions as a physiologically important regulator of the chemotactic activity for C5a, and that this activity is initiated by formation of a novel multi-faceted DBP binding/signaling complex on the cell surface. The objective of this proposal is two- fold. First, the in vivo function of DBP as a C5a chemotactic cofactor will be determined in murine models of C-dependent inflammation using DBP-/- and DBP+/+ mice. This aim will investigate the role of DBP in immune complex mediated (I) peritonitis, (II) alveolitis, (III) alveolitis induced by C-activated serum and purified C5a, and (IV) in vitro using murine leukocytes and purified proteins in a chemotaxis assay. The second specific aim will investigate how DBP enhances chemotaxis to C5a using human neutrophils and U937 cells in vitro. This aim will employ a reverse genetic approach to dissect the process by utilizing (I) mutant DBPs generated by selective deletion of the co-chemotactic and cell binding sequences and (II) RNAi directed at individual members of the DBP binding/signaling complex to silence their expression in U937 cells. Both will be used to determine the effect on DBP-cell binding, calcium signaling, chemotaxis and shedding of the binding/signaling complex. Results of this study will provide a better understanding of how C5a recruits leukocytes in a physiological setting, and also may reveal that DBP could be a target for therapeutic intervention to reduce the leukocyte burden at sites of C activation. PUBLIC HEALTH RELEVANCE: Complement activation peptide C5a is one of the most potent and physiologically important molecules that induce accumulation of white blood cells in tissues (chemotaxis). Excessive or aberrant production of C5a is strongly associated with tissue injury in numerous inflammatory diseases. The vitamin D binding protein (DBP) is a blood protein that has been shown to significantly enhance the chemotactic activity of C5a. This proposal will investigate the mechanisms by DBP regulates the chemotactic activity of C5a in a physiological setting.